Laser Physics PDF - جامعة جنوب الوادي - 2025

‫‪Laser Physics‬‬ ‫جامعة جنوب الوادي‬ ‫فيزياء الليزر‬ ‫‪Laser Physics‬‬ ‫الفرقة الرابعة تربية أساسي لغات‬ ‫شعبة علوم‬ ‫أستاذ...

‫‪Laser Physics‬‬ ‫جامعة جنوب الوادي‬ ‫فيزياء الليزر‬ ‫‪Laser Physics‬‬ ‫الفرقة الرابعة تربية أساسي لغات‬ ‫شعبة علوم‬ ‫أستاذ المقرر‬ ‫د‪ /‬عادل جادالكريم عبادي‬ ‫قسم الفيزياء ‪ -‬كلية العلوم‬ ‫العام الجامعي‬ ‫‪ 2025 / 2024‬م‬ ‫‪1‬‬ Laser Physics Contents Chapter 1 Introduction to Laser Essentials page Topic 5 Electromagnetic Radiation 5 Frequency 6 Wave Description 6 Wavelengths Comparison 8 Electromagnetic Radiation in Matter 9 Bohr model of the atom 10 Energy States (Levels) 11 Photons and the energy diagrams Chapter 2 Basics of laser physics page Topic 14 LIGHT EMISSION AND ABSORPTION 15 QUANTIZATION 15 PHOTONS 17 What is LASER? 17 Properties of LASER 21 Basic components of the laser device 22 How the First Ruby Laser Works 23 The Interaction of Electromagnetic Radiation with Matter 24 Average Lifetime 25 ATOMIC EXCITATION AND EMISSION 25 EMISSION SPECTRA 26 ABSORPTION SPECTRA 27 FLUORESCENCE 28 Light-Emitting Diodes 2 Chapter 3 Laser Physics General Description of LASER and LASER OPERATION 31 LASER OPERATION 31 Stimulated Emission 32 Population Inversion 34 Optical Cavity 35 TYPES OF LASERS Gas Lasers 35 Solid-State Lasers 36 Semiconductor Diode Lasers 37 Common features of lasers Light amplifying media 37 pumping 39 Laser oscillator 43 Optical feedback and laser oscillation 44 The Einstein Relations 54 Three Level Laser and Four Level Laser 58 Laser classifications 59 Fiber Optics Chapter 4 Laser Applications 62 Industrial Applications 63 Medical Applications 66 Military Applications 68 Daily applications 69 Optical Fiber Communications 72 Holograms for exibitions and museums 73 Scientific/Research Applications 75 Special applications 76 Fiber laser 77 Classifications of lasers into groups 83 Glossary of laser definitions 90 References 91 Basic Laser Books 3 Laser Physics 4 ‫‪Laser Physics‬‬ ‫أساسيات فيزياء الليزر و مفاهيم اساسية‬ ‫‪5‬‬ Laser Physics 6 Laser Physics 7 Laser Physics 8 Laser Physics 9 Laser Physics 10 Laser Physics 11 Laser Physics 12 Laser Physics 13 Laser Physics 14 Laser Physics Chapter 1 Introduction to Laser Essentials Before studying about lasers, you must be familiar with basic terms used to describe electromagnetic waves: Wavelength (ʎ) Frequency (ν ) Period (T) Velocity of light (c) Index of refraction (n). We will briefly review these terms, but it is much better if you are familiar with: Some terms from geometric optics such as: refraction, reflection, thin lenses etc. Some terms from "Modern Physics" such as photons, Models of atoms, etc. Electromagnetic Radiation Electromagnetic Radiation is a transverse wave, advancing in vacuum at a constant speed which is called: velocity of light. All electromagnetic waves have the same velocity invacuum, and its value is approximately: c = 300,000 [km/sec] = 3*108 [m/sec One of the most important parameters of a wave is its wavelength. Wavelength (λ) (Lamda) is the distance between two adjacent points on the wave, which have the same phase. As an example (see figure below) the between two adjacent peaks of the wave. distance Frequency In a parallel way it is possible to define a wave by its frequency. Frequency (µ) is defined by the number of times that the wave oscillates per second. Between these two parameters the relation is: c = λ * µ From the physics point of view, all electromagnetic waves are equal (have the sa 15 Laser Physics me properties) except for their wavelength (or frequency). As an example: the speed of light is the same for visible light, radio waves, or x-rays. Wave Description A wave can be described in two standard forms: 1. Displacement as a function of space when time is held constant. 2. Displacement as a function of time at a specific place in space Displacement as a function of space Displacement as a function of space, when time is "frozen" (held constant). In this description, the minimum distance between two adjacent points with the same phase is wavelength (λ). Note that the horizontal (x) axis is space coordinate A = Amplitude = Maximum displacement from equilibrium Displacement as a function of time Displacement as a function of time, in a specific place in space, as described in figure. In this description, the minimum distance between two adjacent points with the same phase is period (T). Note that the horizontal (x) axis is time coordinate 16 Laser Physics Wavelengths Comparison The Figure describes how two different waves (with different wavelengths) look at a specific moment in time. Each of these waves can be uniquely described by its wavelength. The most important ideas summarized infigure are: 1. Electromagnetic waves span over many orders of magnitude in wavelength (or frequency). 2. The frequency of the electromagnetic radiation is inversely proportional to the wavelength. 3. The visible spectrum is a very small part of the electromagnetic spectrum. 4. Photon energy increases as the wavelength decreases. The shorter thewavelength, the more energetic are its photons. 17 Laser Physics Examples for electromagnetic waves are: Radio-waves which have wavelength of the order of meters, so they need big antennas. Microwaves which have wavelength of the order of centimeters. As an example: in a microwave oven, these wavelengths can not be transmitted through the protecting metal grid in the door, while the visible spectrum which have much shorter wavelength allow us to see what is cooking inside the microwave oven through the protecting grid. x-Rays which are used in medicine for taking pictures of the bone structure inside the body. Gamma Rays which are so energetic, that they cause ionization, and are classified as ionizing radiation. Electromagnetic Radiation in Matter Light Velocity in Matter When electromagnetic radiation passes through matter with index of refraction n, its velocity (v) is less than the velocity of light in vacuum (c), and given by the equation: v=c/n (speed of light This equation is used as a definition of the index of refraction n = in vacuum)/(speed of light in matter) n = c/v Gases, including air, are usually considered as having index of refraction equal to vacuum n0=1. The values of the index of refraction of most materials transparent in the visible spectrum is between 1.4-1.8, while those of materials transparent in the Infra- Red (IR) spectrum are higher, and are 2.0- 4.0. Wavelength in Matter We saw that the velocity of light in matter is slower than in vacuum. This slower velocity is associated with reduced wavelength: λ = λ 0/n , while the frequency remains the same 18  Laser Physics Refraction of Light Beam - Snell Law Reducing the velocity of light in matter, and reducing its wavelength, causes refraction of the beam of light. While crossing the border between two different materials, the light changes its direction of propagation according to the Snell Equation THE BOHR MODEL In 1913, Niels Bohr proposed a model of the hydrogen atom that incorporated the quantum aspects of Planck’s hypothesis. Bohr adapted the existing planetary model of the atom, in which electrons revolved around a central nucleus, but added a key assumption: Bohr suggested that the electron’s energy was only allowed to take on particular values and could not be anything in between those values. Furthermore, these energy levels correspond to specific fixed orbits around the nucleus (). A staircase serves as a useful analogy for the energy levels in an atom. Just as it’s not possible for an object to reside in between two steps, an electron cannot occupy a state between two energy levels. Bohr indexed the energy levels of the hydrogen atom according to a quantum number that could take the whole number values n = 1, 2, 3, and so on. The lowest allowable energy level, for which n = 1, is known as the ground state of the atom. In this state, the electron is also closest to the nucleus. Energy levels that are higher than the ground state are called excited states. Electrons must accept energy, for example by absorbing a photon, in order to move from one energy level to a higher one. Only certain frequencies of photons can be absorbed by an individual atomic system, and therefore each energy level diagram is quantized by the allowable transitions between states, which is unique for each type of atom. Hydrogen is the simplest atom in the universe due to its single orbital electron. As a result, physicists often use it as the standard model to explain the basic rules of quantum mechanics and atomic energy levels. In atoms not react or combine easily with other elements. In contrast, alkali metals such as lithium and sodium have an unpaired electron and tend to be highly reactive. There are many more types of energy levels in addition to those found in solitary atoms. Molecules have electronic energy levels of their own, which become more complex as the number of electrons and atoms in the molecule increases. Molecules also have energy levels that depend on vibrations of atoms within them and on the rotation of the entire molecule. All of these energy levels are quantized as well. 19 Laser Physics The Essentials: Atoms Bohr model of the atom Lasing action is a process that occurs in matter. Since matter is composed of atoms, we need to understand about the structure of the atom, and its energy states. We shall start with the semi-classical model, as suggested in 1913 by Niels Bohr, and called: The Bohr model of the atom. According to this model, every atom is composed of a very massive nucleus with a positive electric charge (Ze), around it electrons are moving in specific paths. Z = Number of protons in the nucleus, e = Elementary charge of the electrons: e = 1.6*10-19 [Coulomb] Every "allowed orbit" of the electron around the nucleus, is connected to a specific energy level. The energy level is higher as the distance of the "orbit" from the nucleus increases. Since for each atom there are only certain "allowed orbits", only certain discrete energy levels exist, and are named: E1, E2, E3, etc Energy States (Levels) Every atom or molecule in nature has a specific structure for its energy levels. The lowest energy level is called the ground state, which is the naturally preferred energy state. As long as no energy is added to the atom, the electron will remain in the ground state. When the atom receives energy (electrical energy, optical energy, or any form of energy), this energy is transferred to the electron, and raises it to a higher energy level. The atom is then considered to be in an excited state. The electron can stay only at the specific energy states (levels) which are unique for 20 Laser Physics each specific atom. The electron can not be in between these "allowed energy states", but it can "jump" from one energy level to another, while receiving or emitting specific amounts of energy. These specific amounts of energy are equal to the difference between energy levels within the atom. Each amount of energy is called a "Quantum" of energy (The name "Quantum Theory" comes from these discrete amounts of energy). Photons and the energy diagrams Electromagnetic radiation has, in addition to its wave nature, some aspects of "particle like behavior". In certain cases, the electromagnetic radiation behaves as an ensemble of discrete units of energy that have momentum. These discrete units (quanta) of electromagnetic radiation are called "Photons". The relation between the amount of energy (E) carried by the photon, and its frequency (ν), is determined by the formula (first given by Einstein): E = hν The proportionality constant in this formula is Planck's constant (h): h = 6.626*10-34 [Joule-sec] This formula shows that the frequency of the radiation (ν), uniquely determines the energy of each photon in this radiation. E=hν This formula can be expressed in different form, by using the relation between the frequency (ν) and the wavelength: c = λ*ν to get: E = h * c/ λ This formula shows that the energy of each photon is inversely proportional to its wavelength. This means that each photon of shorter wavelength (such as violet light) carries more energy than a photon of longer wavelength (such as red light). Since h and c are universal constants, so either wavelength or frequency is enough to fully describe the photon. Energy Levels Achieving population inversion and sustaining a chain of stimulated emission events requires a configuration of energy levels with specific characteristics. If the excited atoms undergo spontaneous emission before stimulated emission can take place, there will not be a sustained beam of identical photons traveling in the same direction. For this reason, the excited state must be metastable to ensure that atoms will remain in that state long enough to sustain a population 21 Laser Physics inversion. Some amount of spontaneous emission is unavoidable, but the longer the lifetime of the state, the easier it is to ensure that stimulated emission will dominate. Although two-level lasers are theoretically possible under certain conditions, it is generally unfeasible to excite atoms directly into a metastable state. A more practical approach involves three energy levels: a ground state, an excited state with a short lifetime (relative to the other two transitions), and a metastable state with a slightly lower energy (FIGURE 58). Atoms are excited, or “pumped,” into the higher energy level, where they quickly de-excite to the metastable level. The metastable level is chosen to have a lifetime generally a thousand times longer than that of the higher energy level. This process accumulates a large population of atoms in the metastable state, thereby establishing a population inversion between it and the ground state. A laser is not itself a source of energy; rather, it needs energy from an external source in order to continually maintain population inversion. An external source of energy, called the pump source, excites the atoms in the active medium to the metastable state. Although a variety of techniques have been used in laser pumping, the most common methods are optical pumping and electrical pumping. Optical pumping is the use of an intense light source, such as a flashlamp, arc lamp, or external laser to excite atoms through photon absorption. Electrical pumping involves the use of an electric discharge or current to cause atomic excitation. Three-level lasers are not an optimal solution for laser operation; a more efficient method involves four energy levels. A four-level laser adds an additional energy level above the ground state, which becomes the lower level for the laser emission transition. In other words, the lower level of the laser transition is not the ground state, which makes it easier to maintain a population inversion between the laser transition states. In a four-level laser, the lower level of the laser transition starts out nearly empty because it has a higher energy than the ground state. Thus, only exciting a small fraction of the ground state atoms to the metastable state is sufficient to establish a population inversion. 22 Laser Physics Chapter 2 Introductory Concepts Basics of laser physics LIGHT 23 Laser Physics EMISSION AND ABSORPTION THE ATOM In order to understand how light is emitted, we must first consider the structure and properties of the atom. Atoms are the building blocks of the world around us. Atoms contain a nucleus, which is a dense core that comprises most of the atom’s mass, and an outer region inhabited by bound electrons. The nucleus is made up of positively charged particles called protons as well as neutrons, which have no electric charge. The nucleus is surrounded by negatively charged electrons. These electrons occupy “shells” that have different spatial distributions. Some are spherical, some are barbell-shaped, and others have more complex distributions. Due to the attractive force between oppositely charged particles, the farther an electron is from the nucleus, the more potential energy it has with respect to the nucleus. FIGU RE 39 A substance composed of only one kind of atom is called an element. Each element in the periodic table has a characteristic atomic number, which is equal to the number of protons in the nucleus of an atom of that element. Electrons and protons have the same magnitude, but opposite sign, of electric charge. Because atoms are electrically neutral, they must have the same number of electrons as protons (). (Atoms can gain or lose electrons to become ions, which have an overall negative or positive charge.) Hydrogen, which has an atomic number of 1, contains a single proton in its nucleus, orbited by a single electron. Likewise, all carbon atoms (atomic number 6) contain six protons and six electrons. A neutral atom contains an equal number of protons and electrons. QUANTIZATION You can combine protons and electrons to form atoms using the “Build an Atom” PhET Lasers are fundamentally quantumapp mechanical devices.. That is, the operation of a laser is fully at phet.colorado.edu dependent on the quantum nature of light and matter. Let’s explain what this means. Before 1900, scien 24 Laser Physics tists assumed energy could vary continuously and be endlessly subdivided. No experiment had provided evidence to contradict this viewpoint. In 1900, a German physicist named Max Planck () proposed that the energy could only be emitted or absorbed in discrete bundles that are multiples of a fundamental unit, or quantum, of energy. This relationship can be expressed as E = nhf , where h = 6.63 × 10−34 J s is called Planck’s constant; f is the frequency of oscillation; and n is a positive integer. That is, energy could be exchanged in amounts of hf, 2hf, 3hf, etc., but not anything in between. Because the energy can only take certain specific values, we say that it is “quantized.” As an analogy, imagine dunes of sand that appear smooth from a distance, but are in fact coarse and grainy when observed close up. Planck’s constant is small enough that we do not notice the quantization of energy in our everyday experience—the quantized packets of energy are too small to be individually perceived by our senses. PHOTONS Experiments conducted around the same time as Planck’s hypothesis revealed that light itself exhibits similar quantum behavior. Heinrich Hertz had discovered in 1887 that metals illuminated with ultraviolet light tended to produce sparks. J. J. Thomson later determined that these sparks were actually electrons being ejected from the surface of the metal. This phenomenon became known as the photoelectric effect. Physicists initially attempted to use Maxwell’s electromagnetic wave model to explain the photoelectric effect. They reasoned that since electromagnetic fields exert forces on charged particles, the oscillating fields in a light wave could theoretically push an electron within an atom back and forth as one might push a child on a swing. Eventually, the electron could have enough energy to escape the atom, and the metal, altogether. the photoelectric effect. Light below a certain frequency does not eject electrons, whereas higher-frequency light ejects electrons with more energy. These observations cannot be explained by treating light as a wave. However, further investigation of the photoelectric effect in 1902 resulted in several observations that 25 Laser Physics contradicted physicists’ expectations under the wave model. For instance, electrons were emitted almost instantaneously (less than a nanosecond) after the metal was illuminated, whereas the wave model predicted a time delay. Furthermore, light below a certain frequency was not observed to eject any electrons, regardless of the intensity of the light. According to the existing theory, any frequency of light would be expected to eject electrons if the incident light was sufficiently intense. In addition, increasing the frequency of the incident light resulted in higher-energy electrons being emitted—however, there should have been no relationship between light frequency and electron energy when treating light as a wave (). Drawing from Planck’s theory, in 1905 Albert Einstein proposed that light itself is quantized into iscrete energy packets that exhibit properties of particles. Einstein argued that the photoelectric effect could only be explained as packets of light colliding with electrons, thereby transferring their energy and ejecting the electrons from the metal. These packets of light energy eventually became known as photons. Furthermore, Einstein argued that the energy of these photons obeys Planck’s relation E = hf, which was consistent with experimental results from the photoelectric effect. Thus, an individual photon from a higher-frequency beam of light carries more energy than a photon from a lower-frequency beam of light. Einstein was awarded the Nobel Prize in 1921, in part for his explanation of the photoelectric effect. Photons are packets of energy transferred from the electromagnetic field. Although they have zero mass, photons have both energy and momentum. It takes an extremely large number of photons to make up most of the radiation we experience on a daily basis because the energy of any individual photon in the visible spectrum is so small. For example, a quick estimate can be made that a single 100 watt light bulb will release on the order of 1021 photons every second. Normally we do not “feel” an individual photon any more than we would feel an individual droplet of water while swimming through the ocean. Before we move on, we should address an important point. Although they seem contradictory, the photon, or particle, nature of light does not invalidate the wave nature of light we discussed in the previous section. Light exhibits properties of both waves and particles, depending on the experiment (FIGURE ). This seemingly contradictory behavior is known as wave-particle duality. The photoelectric effect is a demonstration of the particle nature of light, whereas double-slit interference is a wave phenomenon. Wave-particle duality is a puzzling aspect of the physical world that took the world’s leading physicists many years to accept. What is LASER? 26 Laser Physics Light Amplification by Stimulated Emission of Radiation. Light: All light is a form of electromagnetic radiation that is visible to the human eye. Amplification: This is simply the process of making something bigger or more powerful. When you turn up the volume on a radio, you are amplifying the sound; but with lasers, amplification makes the light brighter. Stimulated: To stimulate means to stir to action. Laser light is created when a burst of light (electricity) excites the atoms in the laser to emit photons. These photons then stimulate the creation of additional identical photons to produce the bright laser light. Emission: The word "emission" refers to something that is sent out or given off. Stimulated laser emission consists of large numbers of photons that create the intense laser light. Radiation: The laser light is a form of energy that radiates, or moves out, from the laser source. Properties of LASER Laser light has several properties that make it useful for many practical applications. Laser light is monochromatic, directional, and coherent. By comparison, ordinary white light is a combination of many wavelengths of light, emits isotropically (in all directions), and is a mixture of many out‐ of‐phase wavelengths. These three properties of laser light are what can make it more hazardous than ordinary light— laser light is capable of depositing a lot of energy within a small area. We will discuss each of these properties in greater detail. Monochromatic The light emitted from a laser is monochromatic, it is of one wavelength (color). In contrast, ordinary white light is a combination of many different wavelengths (colors). Figure : White light passing through a prism Beca 27 Laser Physics use the photons emitted by a laser all correspond to the same energy transition, they all have the same frequency. Single-frequency light such as this is often described as monochromatic. In comparison, recall that thermal radiation (such as that produced by an incandescent light source) produces a continuous spectrum of frequencies with different intensities. Laser light is not perfectly monochromatic; there is some “spread” due to Doppler shifts from the motion of atoms or molecules within the active medium. The line width or bandwidth of a laser describes the spread of its spectrum of emitted frequencies. However, this spread is extremely narrow compared to the spectrum of frequencies emitted by, say, an incandescent light bulb. Directional Lasers emit light that is highly directional. It is emitted as a narrow beam in a specific direction. Ordinary light (sun, light bulb, a candle), is emitted in many directions away from the source Figure : comparison between the light out of a laser, and the light out of an incandescent lamp The light from a laser emerges as a very narrow beam with very little divergence, or spread. We often refer to a beam with this property as being collimated. If you’ve used a laser pointer while giving a presentation or distracting a feline companion, you are familiar with the ability of lasers to project a point of light, even from a relatively large distance. A laser’s high degree of collimation is a direct result of the precise alignment of the parallel mirrors that form the optical cavity. As the light waves reflect back and forth many times within the cavity, the mirrors constrain the waves to an axis perpendicular to the surfaces of both mirrors. Any light that is slightly “off-axis” will be lost from the cavity and thus will never form part of the final beam. The highly collimated nature of a laser beam makes it highly useful but also highly dangerous. You should never look directly into a laser beam because the highly parallel rays can focus to a nearly microscopic dot on the retina of your eye, causing almost instant damage to the retina. On the other hand, the ability of lasers to focus so precisely contributes to their wide range of both medical and industrial applications. In medicine, lasers can be used as sharp scalpels; in industry, they can serve as fast, powerful, and computer-controllable cutting tools. 28 Laser Physics Coherence Coherent waves are waves that maintain the relative phase between them Since electromagnetic radiation is a wave phenomena, every electromagnetic wave can be described as a sum (superposition) of sine waves as a function of time. From wave theory we know that every wave is described by a wave function: y = Acos(t+) Amplitude.  = 2Angular Frequency.  = Initial Phase of the wave (Describe the starting point in time of the oscillation). (t+) = Phase of the wave. Coherent Light that is made up of waves that are “in-phase” relative to one another is said to be coherent. In other words, the peaks and troughs of the waves exactly align (FIGURE 61). An ordinary light source, such as an incandescent light bulb, produces light that is incoherent, meaning the waves have random phases. Even a collection of waves with identical frequencies can be incoherent if they are not in-phase relative to one another. LEDs, for example, emit light that is single-frequency but not coherent. Coherence in laser light is a direct consequence of stimulated emission. Coherence in laser light is important for observing interference effects, which has important applications in precision measurement. Interferometry is the use of superimposed waves to make extremely fine measurements of small displacements, surface irregularities, or changes in refractive index. A basic interferometer uses a beam splitter and mirrors to overlap beams of light from a coherent source 29 Laser Physics such as a laser. By slightly adjusting the path length difference between the two beams, shifts in the observed interference pattern will occur as the relative phases of the two beams change. This technique allows measurements to be made on the length scale of the wavelength of light being used. Laser Radiation Properties In Summary: Laser Radiation Properties 1. Very small divergence of the beam. The beam is almost a parallel beam and move in one direction in space - Directionality.. 2. High degree of monochromaticity. The radiation is almost one wavelength, as can be measured by the very narrow spectral width. 3. Coherence. The combination of these properties gives the laser radiation many advantages, like achieving very high power densities, not available from other sources. Coherent waves (top) and incoherent waves (bottom). Each individual wave has the same frequency 30 Laser Physics Basic components of the laser device In order for most laser to operate, three basic conditions must be satisfied (1) The active medium: Collections of atoms, molecules or ions in the form of solid or liquid or gas. (2) population inversion (3) Optical feed back (4) How the First Ruby Laser Works 31 Laser Physics Excitation High-voltage electricity causes the quartz flash tube to emit an intense burst of light, exciting some of the atoms in the ruby crystal to higher energy levels. Photon Emission At a specific energy level, some atoms emit photons. At first the photons are emitted in all directions. Photons from one atom stimulate emission of photons from other atomsand the light intensity is rapidly amplified Amplification Mirrors at each end reflect the photons backand forth, continuing this process of stimulated emission andamplification Laser Beam The photons leave through the partially silvered mirror at one end. This is laser light. The Interaction of Electromagnetic Radiation withMatter Emission and Absorption of Radiation The interactions between electromagnetic radiation and matter cause changes in the energy states of the electrons in matter. Electrons can be transferred from one energy level to another, while absorbing or emitting a certain amount of energy. This amount of energy is equal to the energy difference between these two energy levels (E2-E1). When this energy is absorbed or emitted in a form of electromagnetic radiation, the energy difference between these two energy levels (E2-E1) determines uniquely the frequency (ν) of the electromagnetic radiation: Emission and Absorption of Radiation Every system in nature "prefers" to be in the lowest energy state. This state is called the Ground state. When energy is applied to a system, The atoms in the material are excited, and raised to a higher energy level. 32 Laser Physics (The terms "excited atoms", "excited states", and "excited electrons" are used here with no distinction) These electrons will remain in the excited state for a certain period of time, and then will return to lower energy states while emitting energy in the exact amount of the difference between the energy levels ( E). If this energy is transmitted as electromagnetic energy, it is called photon. Spontaneous Emission The emission of the individual photon is random, being done individually by each excited atom, with no relation to photons emitted by other atoms. When photons are randomly emitted from different atoms at different times, the process is called Spontaneous Emission. Since this emission is independent of external influence, there is no preferred direction for different photons, and there is no phase relation between photons emitted by different atoms. Spontaneous emission is one of a family of processes, called relaxation processes, by which the excited atoms return to equilibrium (ground state). This "classic" explanation assumes that the specific frequencies emitted by an excited atom are the same as the characteristic frequencies of the atom, which means that the emission spectrum is identical to the absorption spectrum Average Lifetime 33 Laser Physics Atoms stay in an excited level only for a short time (about 10-8 [sec]), and then they return to a lower energy level by spontaneous emission. Every energy level has a characteristic average lifetime, which is the average time the electron exists in the excited state before making a spontaneous transition. Thus, this is the time in which the excited atoms returned to a lower energy level. According to the quantum theory, the transition from one energy level to another is described by statistical probability. The probability of transition from higher energy level to a lower one is inversely proportional to the lifetime of the higher energy level. When the transition probability is low for a specific transition, the lifetime of this energy level is longer (about 10-3 sec), and this level becomes a "meta-stable" level. In this meta-stable level a large population of atoms can assembled. As we shall see, this level can be a candidate for lasing process. When the population number of a higher energy level is bigger than the population number of a lower energy level, a condition of "population inversion" is established. If a population inversion exists between two energy levels, the probability is high that an incoming photon will stimulate an excited atom to return to a lower state, while emitting another photon of light. The probability for this process depend on the match between the energy of the incoming photon and the energy difference between these two levels ATOMIC EXCITATION AND EMISSION Each element has a characteristic set of energy levels that are common to every atom of that element. The process of an electron moving from a lower to a higher energy level is called excitation, which occurs when the atom absorbs energy corresponding to a particular energy level transition. This energy input can result from absorbing a photon, but can also come from kinetic energy due to collisions with other particles or thermal energy from being heated. Once they are in an excited state, electrons can drop to lower energy levels by emitting a photon with energy equal to the difference in energy between the two levels. The frequency of the emitted light is given by the Planck relation E = hf ( ). A neon light is a familiar example of a gas-discharge lamp, which produces light as a result of atomic de- excitation (FIGURE ). Gas-discharge lamps consist of glass tubes filled with a noble gas 34 Laser Physics such as argon, neon, or xenon. The color of the lamp depends on which gas is contained within the tube. True neon lights are reddish-orange in color; other colors, such as green or blue, are produced by different gases. At each end of the tube are electrodes that are connected to a voltage source. The voltage causes electrons to leave one of the electrodes and vibrate back and forth within the tube, colliding with millions of atoms as they do so. The collisions with the electrons transfer energy to the atoms, exciting their orbital electrons to higher energy levels. The atoms emit a characteristic photon as they de-excite, and the process continues. FI FI G G U U RE R 44 E 43 EMISSION SPECTRA The characteristic set of wavelengths emitted by a collection of excited atoms is called an emission spectrum. Emission spectra can be viewed with a measurement device called a spectroscope, which uses a prism or grating to separate light emitted by a collection of excited atoms into component wavelengths. FIGURE 46 shows a common configuration for a spectroscope, in which the emitted light is passed through a thin slit and projected onto a viewing screen. Each component color is refracted to a definite The Bohrposition onthe model of thehydrogen screen, forming a distinct image of the slit as a narrow line. These differently a colored lines are often called spectral lines. t FIGURE shows emission spectra for an incandescent source, a hydrogen lamp, and a collection o of excited iron atoms. The emission spectrum for hydrogen contains four distinct lines in the m visible spectrum, which occur as a result of a hydrogen atom transitioning to the n = 2 state from a An atom emits a photon when an higher energy level. This set of emissioni lines is known as the Balmer series. Other sets of exci hydrogen emissiontedlines outside the visible n spectrum include the Lyman series (which involves transitions to n = 1) and the Paschen series elec c (n = 3). l tronan atom contains, the more complicated the energy level structure becomes. The more electrons dro lines are relativelyu The hydrogen spectral d simple, containing only a few visible lines; this is ps because hydrogen contains only a single electron. e to a low atomic numbers, sthe electrons interact with one another and with the For atoms with larger nucleus, creating manyer more energy levels and thus many more possible transitions. Iron, which ene f electrons. Notice that iron has many more visible has an atomic number of 26, contains 26 i spectral lines than rgy hydrogen due to its more complex electron configuration. leve x l. e Since The d 35 ene rgy o of r the b Laser Physics each element has a characteristic set of energy levels, the emission spectrum is also unique to each element. We can therefore think of these spectral lines as a sort of “fingerprint” that indicates the presence of a specific element. The process of analyzing spectral lines to identify the chemical makeup of an excited sample is called atomic spectroscopy. Atomic spectroscopy came into its own as a chemical analysis technique in the late 1800s. The probability that a particular transition will occur depends on the population of the two states and their quantum characteristics. Certain types of quantum transitions are far more likely than others, so each transition to a lower energy state has its own characteristic lifetime, or average length of time for an electron to undergo a transition between those two states. As we will see, these lifetimes are an important factor in laser operation. Complex interactions between adjacent atoms in liquids or solids blur energy levels so that simple absorption and emission lines are not visible. This creates significant differences between gas and solid-state lasers. ABSORPTION SPECTRA Electrons can move to higher energy levels by absorbing electromagnetic radiation. An absorption spectrum is created by passing a continuous spectrum of electromagnetic waves through a collection of atoms. What emerges from the atoms is a continuous spectrum except for black lines, called absorption lines, where specific wavelengths of light have been absorbed. FIGURE 50 shows a segment of the emission and absorption spectra for hydrogen. Notice that the positions of absorption lines correspond with the emission lines for the same element. The Sun is an incandescent source of light, yet if we zoom in to the emission spectrum of sunlight we find that it is not perfectly continuous—instead, there are thin lines where certain frequencies of light are missing. These absorption lines are known as Fraunhofer lines after Joseph von Fraunhofer who first discovered them and determined their wavelengths. Fraunhofer lines occur because the light emitted from the body of a star is absorbed by the atmosphere of cooler gases that surround it. In 1868, scientists deduced that a set of then-unknown spectral lines from the Sun belonged to a yet undiscovered element, which they called helium. Thus, solar atomic spectroscopy revealed helium to be a new element almost three decades before it was first found on Earth. Spectral lines from stars also provide evidence of how fast they are moving relative to us. If you’ve ever stood near a train as it passes, you may have experienced how the sound of the whistle seems to become lower in pitch at the moment the train moves past your position. Although the frequency of the whistle has not changed, the sound waves that reach your ears are shifted lower in frequency. This frequency shift due to a moving source, known as the Doppler effect, also applies to electromagnetic waves. Because we know the exact frequencies of spectral lines due to measurements here on Earth, we can compare them with spectral lines from distant objects to determine the relative shift. If the spectral lines are shifted toward the red side of the spect 36 Laser Physics rum (i.e., lower in frequency), we know the star or galaxy is moving away from Earth. Nearly all galaxies we observe exhibit a red shift in their spectral lines, which serves as evidence that the universe is expanding. FLUORESCENCE Different de-excitation pathways can cause atoms to emit different combination of photons. Fluorescence occurs when a material absorbs a photon and then de-excites by emitting a photon with a lower frequency (i.e., energy). Returning to the staircase analogy we explored earlier, imagine leaping to the top of a small staircase in a single jump. You could get back to the bottom by stepping down each lower step one by one, or by skipping over a step and then taking a smaller step. Likewise, an atom excited by high-frequency (and thus higher-energy) ultraviolet light can take smaller steps to a lower energy state by emitting Figure: Intensity distributions of emitted electromagnetic waves for objects at 3000 K, 4000 K, and 5000 K. Notice how the peak wavelength becomes shorter as the temperature of the object increases. An energy level diagram of fluorescence. After absorbing a photon, an atom emits a photon with a different frequency by dropping down two energy levels in a row. Light-Emitting Diodes A diode is an electronic device that allows electric charge to pass through it in only one direction. Diodes serve many different functions in a variety of electronic devices we rely on every day. For example, diodes are commonly used in power supplies to convert alternating current (AC) to direct current (DC). A photodiode is another type of diode that produces an electric current when light is incident upon it, which is essential to the operation of solar cells. Light-emitting diodes (LEDs) work like photodiodes in reverse; they emit light when an electric current is passed through them. An LED consists of a junction between two semiconductor layers. A semiconductor is a material that can be made to conduct electricity under some conditions but not others. One semiconductor layer (the “n-type”) contains excess electrons, whereas the other layer (the “p-type”) contains “holes” where electrons could be accepted. A barrier prevents the electrons from moving across 37 the  Laser Physics boundary. If a voltage (from a battery, for instance) is placed across the two semiconductor layers, the electrons will be energetic enough to cross the barrier and fill the holes in the adjoining layer. In doing so, the electrons lose energy in the same manner as atomic electrons dropping to a lower energy level. This energy is released in the form of a photon of visible light (FIGURE ). The color of the emitted light is determined by the depth of the energy “holes,” which is a property of the elements used to create the semiconductor layers. A larger drop in energy will correspond to a higher energy photon (i.e., more blue) whereas a lower drop will correspond to a lower energy photon (i.e., more red). The first light-emitting diodes (LEDs) were developed in the 1960s and were only capable of emitting red light. In the 1990s, LEDs of many different colors became widely available. Today, LEDs are used in a wide variety of lighting applications in commercial, industrial, and residential settings. LED arrays are commonly found in traffic lights, automobile brake lights, and electronic billboards and displays. Within the last few years, LED light bulbs have supplanted CFLs as the consensus solution for energy- efficient lighting in most applications. LED bulbs have long lifetimes—approximately five times that of a CFL bulb, and forty times that of an incandescent bulb. Furthermore, LEDs consume less energy than other bulb designs and do not have disposal risks. LEDs also do not undergo abrupt failure as incandescent bulbs do; instead they very gradually decrease in brightness over many hours of use. Fig A light-emitting diode emits photons when excess electrons from an n-type semiconductor layer fill holes in a p-type semiconductor layer. 1Introductory Concepts In this introductory chapter, the fundamental processes and the main ideas behind laser operation are introduced in a very simple way. The properties of laser beams are also briefly discussed. The main purpose of this chapter is thus to introduce the reader to many of the concepts that will be discussed later on, in the book, and therefore help the reader to appreciate the logical organization of the book. 1.1. SPONTANEOUS AND STIMULATED EMISSION, ABSORPTION To describe the phenomenon of spontaneous emission, let us consider two energy levels, 1 and 2, of some atom or molecule of a given material, their energies being E1 and E2.E1 < E2/ (Fig. 1.1a). As far as the following discussion is concerned, the two levels could be any two out of the infinite set of levels possessed by the atom. It is convenient, however, to take level 1 to be the ground level. Let us now assume that the atom is initially in 38 Laser Physics level 2. Since E2 > E1, the atom will tend to decay to level 1. The corresponding energy difference, E2-E1, must therefore be released by the atom. When this energy is delivered in the form of an electromagnetic (e.m. from now on) wave, the process will be called spontaneous (or radiative) emission. The frequency v0 of the radiated wave is then given by the well known expression where h is Planck’s constant. Spontaneous emission is therefore characterized by the emission of a photon of energy hv0 = E2 -E1, when the atom decays from level 2 to level 1 (Fig. 1.1a). Note that radiative emission is just one of the two possible ways for the atom to decay. The decay can also occur in a nonradiative way. In this case the energy difference E2 - E1 is delivered in some form of energy other than e.m. radiation (e.g. it may go into kinetic or internal energy of the surrounding atoms or molecules). This phenomenon is called non-radiative decay. FIG. 1.1. Schematic illustration of the three processes: (a) spontaneous emission; (b) stimulated emission; (c) absorption. Let us now suppose that the atom is found initially in level 2 and that an e.m. wave of frequency (i.e., equal to that of the spontaneously emitted wave) is incident on the material (Fig. 1.1b). Since this wave has the same frequency as the atomic frequency, there is a finite probability that this wave will force the atom to undergo the transition 2 ! 1. In this case the energy difference E2 - E1 is delivered in the form of an e.m. wave that adds to the incident one. This is the phenomenon of stimulated emission. There is a fundamental difference between the spontaneous and stimulated emission processes. In the case of spontaneous emission, the atoms emits an e.m. wave that has no definite phase relation with that emitted by another atom. Furthermore, the wave can be emitted in any direction. In the case of stimulated emission, since the process is forced by the incident e.m. wave, the emission of any atom adds in phase to that of the incoming wave and along the same direction. Let us now assume that the atom is initially lying in level 1 (Fig. 1.1c). If this is the ground level, the atom will remain in this level unless some external stimulus is applied to it. We shall assume, then, that an e.m. wave of frequency is incident on the material. 39 Laser Physics In this case there is a finite probability that the atom will be raised to level 2. The energy difference E2 - E1 required by the atom to undergo the transition is obtained from the energy of the incident e.m. wave. This is the absorption process. To introduce the probabilities for these emission and absorption phenomena, let N be the number of atoms (or molecules) per unit volume which, at time t, are lying in a given energy level. From now on the quantity N will be called the population of the level. For the case of spontaneous emission, the probability for the process to occur can be defined by stating that the rate of decay of the upper state population, (dN2 / dt)sp, must be proportional to the population N2. We can therefore write where the minus sign accounts for the fact that the time derivative is negative. The coefficient A, introduced in this way, is a positive constant and is called the rate of spontaneous emission or the Einstein A coefficient (an expression for A was in fact first obtained by Einstein from thermodynamic considerations). The quantity is called the spontaneous emission (or radiative) lifetime. Similarly, for non-radiative decay, we can often write where is referred to as the non-radiative decay lifetime. Note that, for spontaneous emission, the numerical value of depends only on the particular transition considered. For non-radiative decay, depends not only on the transition but also on the characteristics of the surrounding medium. We can now proceed, in a similar way, for the stimulated processes (emission or absorption). For stimulated emission we can write where.dN2=dt/st is the rate at which transitions 2 1 occur as a result of stimulated emission andW21 is called the rate of stimulated emission. Just as in the case of the A coefficient defined by Eq. (1.1.2) the coefficient W21 also has the dimension of.time-1. Unlike A, however,W21 depends not only on the particular transition but also on the intensity of the incident e.m. wave. More precisely, for a plane wave, it will be shown that we can write 40 Laser Physics Note also that the fundamental processes of spontaneous emission, stimulated emission and absorption can readily be described in terms of absorbed or emitted photons as follows 41 Laser Physics 42 Laser Physics 43 Laser Physics 44 Laser Physics 45 Laser Physics 46 Laser Physics 47 Laser Physics 48 Laser Physics 49 Laser Physics 50 Laser Physics Chapter 3 General Description of LASER and LASER OPERATION 51  Laser Physics LASER OPERATION A laser is a device that emits a narrow beam of single- wavelength, coherent light as a result of stimulated emission. The term laser began as an acronym for “Light Amplification by Stimulated Emission of Radiation.” The output power of laser light can vary from a few thousandths of a watt (in the case of laser pointers) to several thousand watts (industrial laser cutters). Every laser contains an active medium, which is the source of atoms that will undergo cycles of excitation and de-excitation to release photons that will form the laser beam. Depending on the type of laser, the active medium can be a solid, liquid, or gas. Let’s walk through the necessary conditions for laser operation, all of which build upon the principles of energy levels, excitation, fluorescence, and phosphorescence that we covered earlier in this section. Stimulated Emission We have previously discussed how an atom can become excited from a lower to a higher energy level by absorbing a photon with energy equal to the difference between the energies of two levels. Eventually, the atom will return to a lower energy state by emitting a photon in a process known as spontaneous emission. Although we can predict on average how long it will take for de-excitation to occur (i.e., the lifetime of the state), it’s impossible to predict exactly when a specific excited atom will spontaneously emit a photon. Furthermore, the direction of the emitted photon is also random. Atoms can also transition to a lower energy level through a process called stimulated emission. While an atom is in an excited state, the oscillating electric field from a passing photon (at the same frequency as—or very close to—the transition frequency of the excited electron) can cause the atom to emit a second photon that oscillates in precise synchrony with the first (FIGURE ). Most significantly, the emitted photon is identical in frequency, phase, and direction to the first photon. These identical photons are what make up a laser beam. Fig: Stimulated emission occurs when a photon triggers a de-excitation event. FIGUR E 56 Po 52 Stimulated emission occurs when a photon triggers a de-excitation event. Laser Physics pulation Inversion A functional laser depends on not just a handful of stimulated emission events, but rather a full cascade that will continually generate a stream of stimulated emission photons. When a collection of atoms is in thermodynamic equilibrium (that is, it is not exchanging energy with its surroundings), the vast majority of the atoms tend to be in the lowest possible energy state. This poses a problem because an emitted photon is almost certain to be absorbed by an atom in the ground state instead of stimulating emission in an excited atom. Clearly, in order to sustain the chain reaction of stimulated emission events, the atoms must be prepared such that more are in an excited state than are in the ground state. This condition is known as population inversion (FIGURE 57). Stimulated emission will continue as long as population inversion exists within the active medium, but it will slow down and eventually stop if a majority of the atoms are no longer in the higher-energy state. Fig: A population inversion exists when more atoms are in an excited state than in the ground state. FIGU RE 57 Photon Absorption Photon Absorption: A photon with frequency ν 12 hits an atom at rest (left), and excites it to higher energy level (E. 2) while the photon is absorbed. 53 Laser Physics Spontaneous Emission Spontaneous emission of a photon: An atom in an excited state (left) emits a photon with frequency ν12 and goes to a lower energy level (E1). Stimulated Emission Stimulated emission of a photon: A photon with frequency ν 12 hit an excited atom (left), and cause emission of two photons with frequency ν 12 while the atom goes to a lower energy level (E1). Stimulated Absorption We saw that the process of photon absorption by the atom is a process of raising the atom (electron) from a lower energy level into a higher energy level (excited state), by an amount of energy which is equivalent to the energy of the absorbed photon. 54 Laser Physics Stimulated Emission The incoming photon is an electromagnetic field which is oscillating in time and space. This field forces the excited atom to oscillate with the same frequency and phase as the applied force, which means that the atom can not oscillate freely, but is forced to oscillate coherently with the incoming photon Remember that two photons with the same wavelength (frequency) have the same energy: E = hν = hc/ν The incoming photon does not change at all as a result of the stimulated emission process. As a result of the stimulated emission process, we have two identical photons created from one photon and one excited state. Thus we have amplification in the sense that the number of photons has increased. Optical Cavity Once population inversion is achieved (i.e., most of the atoms are in an excited state), a single atom undergoing de-excitation emits a photon that causes a chain reaction of stimulated emission events: that photon causes stimulated emission in a nearby atom, which emits an identical photon, which stimulates the emission of another photon, and so on. In order to sustain the chain of stimulated emission events and amplify the laser beam, the active medium must be surrounded by an optical cavity (also known as a laser cavity or optical resonator). The simplest optical cavity consists of two parallel mirrors, one of which is slightly transmitting to allow the output of the laser beam. One mirror is coated to be completely reflective (100 percent chance of reflection) whereas the other is coated to be partially reflective (~95 percent chance of reflection). Photons that “leak” from the partially reflective mirror form the laser beam. It’s possible for light to reflect back and forth several hundred times before exiting the resonator ( ). An optical resonator is essential to amplify emitted light waves while allowing a fraction of the photons to escape, forming the output beam. 55 Laser Physics 56 Laser Physics 57 Laser Physics TYPES OF LASERS Gas Lasers Gas lasers use a low-pressure gas mixture as an active medium. Most gas lasers are excited by passing an electric current through the gas, delivered by electrodes placed at opposite ends of the tube. The helium-neon laser (or HeNe laser) is a common gas laser that produces light in the visible spectrum at a wavelength of 632.8 nm. HeNe laser cavities contain a mixture of helium and neon gas. The helium atoms are excited by an applied current and then collide with neon atoms to excite them to the state that causes the 632.8 nm radiation. The bright red output and relatively low cost of HeNe lasers make them well suited for many low-power applications in educational and research laboratories (FIGURE ). Other examples of gas lasers include the carbon dioxide (CO2) laser, which is a high- efficiency laser that operates in the infrared band. Carbon dioxide lasers are commonly used for high-power industrial applications such as welding and cutting. Excimer lasers are a type of gas laser that rely on the excitation of “dimer” molecules, such as argon fluoride, that are stable only in the excited state. Excimer lasers were first demonstrated in the mid-1970s and are capable of removing extremely fine layers of surface material by breaking molecular bonds without burning or heating the surrounding area. For this reason, excimer lasers are well- suited for precision etching of plastics or semiconductor circuits, as well as delicate eye surgery such as LASIK. An advantage of gas lasers over other laser types is that the gas medium tends to be both relatively inexpensive and largely resistant to damage. However, gas lasers are also typically larger than other types due to the low density of the medium. In recent years, gas lasers have seen a decline in sales as they have gradually been replaced by solid-state and semiconductor lasers for many commercial applications. For example, HeNe lasers were originally used in grocery store checkout scanners, but have largely been replaced by laser diodes for this purpose. Solid-State Lasers Solid-state lasers use an active medium consisting of a solid crystalline or glass rod (known as the host) containing light-emitting atoms (the active species). The first laser ever built was a solid-state laser using synthetic ruby, which is corundum (aluminum oxide crystal) with chromium as an active species. In most solid-state laser materials, the active species is identified first (typically by its chemical symbol), followed by the host material. For example, the Ti:sapphire laser consists of titanium atoms in sapphire crystal. Both the active species and the host are important in solid-state lasers. The active species determines the laser transition, but its interactions with the host may shift the wavelength slight 58 Laser Physics ly.Neodymium (Nd) is commonly used as an active species in solid-state laser crystals. For example, the Nd:YAG (neodymium-yttrium aluminum garnet) is one of the most common types of laser, with applications in research, medicine, manufacturing, and other fields ( ). Nd:YAG lasers typically emit infrared light with a wavelength of 1064 nm although other wavelengths are possible. Other host materials for neodymium include YLF (yttrium lithium fluoride) and glass. The host material is selected based on its optical, thermal, and mechanical properties. Semiconductor Diode Lasers Semiconductor diode lasers, commonly known as laser diodes, operate using the same basic principles as light- emitting diodes, but with some important differences. Like an LED, a laser diode consists of two semiconductor layers, an n-type with an excess of electrons and a p-type with electron “holes” to be filled. The semiconductor layers are separated by a microscopic region called the active layer that serves as the optical resonator. Laser diodes operate at much higher currents than LEDs, typically around ten times greater. Whereas an LED emits photons in all directions from its junction layer, a laser diode is configured with reflective ends to form an optical resonator in the region between the semiconductor layers. In a laser diode, stimulated emission occurs when a photon emitted by one electron transition triggers another electron to fill a hole, and so on, resulting in a coherent beam of light that emerges from one side of the diode. Laser diodes are compact and easy to mass-produce. In terms of sheer numbers, they are the most common type of laser. Their small size makes them well suited for use in low-power applications such as laser pointers, laser printers, and CD/DVD players (FIGURE 64). Laser diodes can also be operated at lower voltages than other types of lasers. While gas and solid- state lasers require input voltages on the order of kilovolts, laser diodes can be operated at only a few volts. Laser diodes are typically not as collimated as beams from other types of lasers. In many cases, an external lens is used to correct the shape of the beam, which contributes to the overall fragility of the laser since damage to the lens could render it non-functional. Furthermore, the delicate nature of the semiconductors makes laser diodes more sensitive to static discharges and currents. Excess electrical current can cause the diode to become inoperable. Laser diodes can also degrade in power efficiency over time, gradually requiring more power to output the same beam intensity. 59 Laser Physics 60 Laser Physics 61 Laser Physics 62 Laser Physics 63 Laser Physics 64 Laser Physics 65 Laser Physics 66 Laser Physics 67 Laser Physics 68 Laser Physics 69 Laser Physics 70 Laser Physics 71 Laser Physics 72 Laser Physics 73 Laser Physics 74 Laser Physics 75 Laser Physics 76 Laser Physics 77 Laser Physics Exercises 1- Find the wavelength at which the rate of spontaneous emission is equal to the stimulated emission at a temperature under the condition of thermal equilibrium. 2 - Explain mathematically that there is no laser beam generation when the thermal energy is equal the photon energy. Planck's constant and Boltzmann's constant 3 - What is the temperature for the occurrence of the laser action and laser generation? 4- Calculate the ratio between the spontaneous emission and stimulated emission of a tungsten lamp due to a heat T = 1727 C , where the light is visible. 78 Laser Physics laser classification: A - Continuity of radiation: continuous or pulsed. The pulsed laser emits its beam in the form of a series of very short light pulses. These pulses are issued only when the active medium is in its highest excited state. Some types of lasers emit their beams at a rate of one pulse every few minutes. There are types of lasers, such as the carbon dioxide laser, whose waves can be pulsed or continuous. b - Radiation frequency: visible light, ultraviolet rays, infrared rays, X-ray lasers. ‫أ‬ 79 Laser Physics Any laser beam generator device consists of the following: Three Level Laser A schematic energy level diagram of a laser with three energy levels is shown in figure. The two energy levels between which lasing occur are: the lower laser energy level (E1), and the upper laser energy level (E2). To simplify the explanation, we neglect spontaneous emission. To achieve lasing, energy must be pumped into the system to create population inversion. So that more atoms will be in energy level E2 than in the ground level (E1). Atoms are pumped from the ground state (E1) to energy level E3. They stay there for an average time of 10-8 [sec], and decay (usually with a non-radiative transition) to the meta-stable energy level E2. Since the lifetime of the meta-stable energy level (E2) is relatively long (of the order of 10-3 [sec], many atoms remain in this level. If the pumping is strong enough, then after pumping more than 50% of the atoms will be in energy level E2, a population inversion exists, and lasing can occur. 80 Laser Physics Figure: Energy level diagram in a three level laser Four Level Laser The schematic energy level diagram of a four level laser is shown in figure. Compared to the equivalent diagram of a three level laser, there is an extra energy level above the ground state. This extra energy level has a very short lifetime. The pumping operation of a four level laser is similar to the pumping of a three level laser. This is done by a rapid population of the upper laser level (E3), through the higher energy level (E4). The advantage of the four level laser is the low population of the lower laser energy level (E2). To create population inversion, there is no need to pump more than 50% of the atoms to the upper laser level. The population of the lower laser level (N2(t)) is decaying rapidly to the ground state, so practically it is empty. Thus, a continuous operation of the four level laser is possible even if 99% of the atoms remain in the ground state ( ) 81 Laser Physics Figure: Energy level diagram in a four level laser 82 Laser Physics 83 Laser Physics 84 Laser Physics Laser classifications risk ‫إشارة تحذير بوجود ليزر‬ Are classified as types of lasers in accordance with the laws of toxic in international standards based on the degree of harm to the human body and must be recalled that the more the resulting damage from the use of the laser is not because of the rays, but because of the misuse of sources of energy crisis for some special devices laser large that generating devices power high-voltage or materials harmful chemicals to humans. As for the damage resulting from its rays, it is mostly on the user's eye, and this does not mean that it is not dangerous to other organs. The damage that the laser may cause to the human eye depends on the following: 1 - The duration of exposure to radiation. 2 - Intensity of radiation. 3 - The color of the laser (or what is known as the wavelength). Fiber Optics Whenever people talk about telephone or television systems that operate with terrestrial cables or Internet networks, the conversation is always associated with mentioning fiber optics, so what are optical fibers. Optical fibers are long filaments of high-purity glass that are as thin as a human hair. These hairs are lined up together in a bundle called an optical cable. If you look 85 Laser Physics closely at one of these optical fibers, you will find that it consists of: The Core molding is an ultra-clear glass core that represents the path through which light travels. The glass shell is cladding, which is the outer material that surrounds the glass heart, and it is made of glass whose refractive index differs from the refractive index of the glass from which the heart is made and constantly reflects light to remain inside the glass mold Buffer coating is a plastic covering that protects the heart from damage Hundreds or perhaps thousands of these optical fibers are lined up together in a bundle to form an optical cord that is protected by an outer covering called \ 86 Laser Physics 87 Ch Laser Physics apter 3 Laser Applications 3 Laser Applications. The number of applications of lasers is enormous, and it is not possible to explain all of them here. In this chapter, the applications are divided into groups, and our hope is that with time we will fill the missing information on most of the well known applications of lasers. Some applications are already described in details, such as: 3.1 Industrial Applications Industry accepted the laser as a tool soon after the laser was invented in 1960. At first the laser was used for alignment and measurements, but with time applications using high power laser beams became more common. The main industrial applications are: 3.1.1 Accurate measurements (Distance, Movement, Interferometry). Since laser radiation is electromagnetic radiation, traveling at the speed of light, very accurate measurements can be performed with lasers. Because of its high speed (the speed of light (c) is the ultimate speed …), measurements of high speed moving objects is not a problem, and the information is available in (almost) real time.  Measurement of the distance from Earth to the moon: One of the known precise measurements with a laser was measuring the distance from Earth to the moon. The astronauts who landed on the surface of the Moon left there a corner cube (a system of three perpendicular mirrors that reflect light in the same direction where it came from). A pulsed laser beam was sent from Earth to the moon and was reflected from this corner cube back to Earth. The travel time of the pulse was recorded. From the known speed of light (c) the distance was calculated, with accuracy of tens of centimeters (!). 88 Laser Physics 3.1.2 Straight line marking, or plan of reference. Many daily applications require a precise reference line for alignment. Examples are:  Laying pipes of gas, water, electricity, etc.  Digging tunnels under-ground (such as the one under the English Channel between England and France).  Alignment of mechanical systems.  Marking spots for pointing invisible radiation from another laser (such as Nd- YAG or CO2 lasers). The visible laser radiation is aligned parralel to the invisible radiation, such that it mark the place where the invisible beam is pointing.  Marking a reference plane for construction: By using a vibrating (or rotating) mirror to reflect a visible laser light, a perfect plane is defined in space. The mirror is vibrating around one axis, so the light is reflected into consecutive angles continuously, thus defining a perfect plane. Since the vibration of the mirror is at a frequency greater than the persistence of vision in the brain, the viewer see a plane of light. This plane helps aligning walls, sealing, etc. in industrial construction. 3.1.3 Material working The main advantages of lasers for material processing are:  Very high accuracy in the final processed products that can be obtained without the need for polishing.  No wearing of mechanical tools. Mechanical tools change their dimensions during the working process, and require constant measurements and feedback to adapt their position to original plan in computerized instrumentation. Material processing include many kinds of processes. A partial list include:  Cutting - The laser can be a very precise cutting tool. High power lasers are used for cutting steel, while other lasers are used to cut fabrics, rubber, plastic, or any other material.  Welding - Combining (fusing) two materials together. By heating the materials near the connecting region, the materials melt locally, and fuse together.  Hardening - By heating specific areas of the material, most metals can be hard 89 Laser Physics ened most of the metals. Even local hardening of specific part of a tool can be done by local irradiation.  Melting - Absorption of laser beams caused a rise in temperature. Since very high power can be transferred to materials in a very short time, melting can be easily done.  Evaporating - Used to ablate material (transfer it into the gas phase).  Photolithography - specially in the semiconductor industry. Very delicate shapes can be created in materials which are used for masks in photolithography. Special materials respond to light at specific wavelength by changing their properties. Thus it is possible to remove parts of the material with very high precision (in micrometer range).  3-D Laser measurements - With the help of a scanning laser, it is possible to obtain the information about a shape of a three-dimensional object and put it in the computer.  3-D Stereo lithography - Similar to photolithography, but the laser is used to create three dimensional sculpture of the information stored within a computer. A combination of the last two applications enable creating 3-D models. Even statue of people were build with high accuracy using these techniques. 3.1.4 Spectral analysis. We saw in chapter 2, and chapter 5, that the entire lasing process is based on absorption and emission of photons at certain specific wavelengths. The wavelength emitted from the laser is monochromatic, and its linewidth is very narrow. Thus, the laser can be used for controlled excitation of molecules. Especially useful for this are the tunable lasers, whose wavelength can be precisely tune to excite specific molecule. 3.2 Medical Applications There are many medical applications of lasers, and there are different ways to classify them into groups: According to the organ to be treated by the laser, such as: Eye, General Surgery, Dentistry, Dermatology, Blood vessels, Cardiac, etc. According to the type of laser used for treatment, such as: CO2, YAG, and Argon. According to the type of treatment, such as diagnostic, surgery, connecting blood vessels. The classification used here is basically according to the type of treatment, with comments on suitable lasers used for each application: 90 Laser Physics 3.2.1 Lasers in medical surgery. 3.2.2 Lasers in diagnostic medicine, and in combination with drugs. 3.2.3 Lasers for specific applications: Soft lasers. When using lasers for medical treatments, a good understanding of the interaction between specific laser radiation with specific biological tissue is required. 3.2.1 Lasers in Medical Surgery Almost every medical surgery in which a removal of tissue is required or a cut needs to be made, can be done with a laser. In general, the results of surgery using lasers are better than the results using a surgical knife. The Advantages of Laser Surgery:  Dry field of surgery, because laser energy seals small blood vessels.  Less postoperative pain, because of the sealing of nerve ends.  No contact with mechanical instruments, so sterilization is built in.  Clear field of view, because no mechanical instrument blocks it.  Possible wavelength specific reaction of specific colors of biological tissue.  Possibility to perform microsurgery under a microscope. The laser beam passes through the same microscope.  Possibility to perform surgical procedures inside the body without opening it, using optical fibers to transmit the laser beam.  The laser can be used as a precise cutting tool.  It can be controlled by a computer, and operate with a very small area of effect under a microscope. 3.2.2 Lasers in Diagnostic Medicine, and in combination with Drugs: Diagnostics of cancer cells using Fluorescence, and Photo Dynamic Therapy (PDT) One of the biggest problems in medicine today is to find a cure for cancer. There are many treatments for cancer to destroy the cancer cells, such as: Disectomy of the infected organ. Radioactive irradiation. Heat treatment. All these treatments improve the chance of cure in some cases, but the "magic" medicine has not yet been found. Since there is no solution yet, the medical professionals are looking for new ways to solve the big problem of cancer. 91 Laser Physics 3.2.3 Soft lasers Most of the medical laser applications were until recently based on the thermal effects caused by the electromagnetic radiation which was absorbed in the biological tissue. In the last few years, some new applications are using low power lasers with output power less than 1 Watt. Some of the effects of these low power levels on the biological tissue is not thermal, and in effect the mechanism of interaction is not yet clear. It is sometimes referred to as Biostimulation, which does not explain a lot. 3.3 Military Applications Since the invention of the laser, its potential military uses were exploited. Large number of projects on lasers were done in secret laboratories, and many years passed until the public was notified about these projects. In the last few years, with the fall of the "Iron Curtain", and the creation of collaboration between the super-powers, the public found about some of these big projects that cost so much money. We shall concentrate on some of the simplest and most known applications, such as: 3.1 Laser Range-finder Measuring distances with high speed and high accuracy was the immediate military application after the laser was invented. Since the laser beam is electromagnetic light, it is traveling in space with known velocity (the velocity of light c). By sending a short laser pulse to the target and measuring the time it take the beam to arrive at the target and reflect back to the sender, it is easy to calculate the distance. Measuring distances with high accuracy is important for military applications such as: Measuring the distance to a shooting target for artillery and missiles. Navigation. Numerical Example: How much time will the laser pulse travel, when it hit a target at a distance of 1.5 kilometer? t = s/c = 3,000 [m]/3*108 [m/s] = 10-5 [sec] = 10 [micro-sec] This time is well within the response time of standard electronic equipment. 3.3.2 Laser Target Designator The laser is used to mark targets for attack by "smart" artillery and guided mis 92 Laser Physics siles. The properties that make the laser so attractive as laser designator are: The laser beam advance great distances in a straight line. The laser beam propagate at very high speed (speed of light). It is possible to modulate the laser beam to include information for identification. A soldier in the field, or a flying vehicle can be used to send a laser beam on the target. The laser is designed to send a series of pulses in a specific pattern (code) of pulses of invisible light. Special detecting systems are locked on these specific pattern of laser pulses, and guide the "Smart Bombs" to hit the marked target. An example showing several laser designator system are shown in figure 9.1. Figure 9.1: Laser Designator systems in the Battlefield. 3.3.3 Laser weapons ("Star War"). A lot was written on the Strategic Defense Initiative (SDI) of the US government. This futuristic project was named by the public "Star Wars". The idea behind this initiative was to build high power devices that can send beams over very big distances in a very high accuracy and very high speed. These high power devices were supposed to destroy the USSR missiles above their lounching sites right after this launch. Since these missiles were supposed to carry nuclear weapons, it was not possible to let them arrive above Europe or the US. By destroying the missiles at the launch zone a great damage would be caused to the attacker, so such defense system was a threat to the other side. 93 Laser Physics 3.3.4 Laser blinding for man and sensitive equipment. A simple and very promising project, which is being developed at many sites all over the world, is laser system for blinding enemy soldiers and their optical equipment. The power required is not specially high, because of the high sensitivity of our sight system, and the high sensitivity of the optical detection systems in use at the battlefield. The operation of blinding laser system is simple: The laser beam is used to scan the space in front of the military troops, blinding enemy soldiers and their equipment. As can be seen in the Appendix, optical power density higher than the safe level can cause blindness (temporary or permanent) to humans, and saturation or damage to sensitive optical equipment. 3.4 Daily applications Since the daily applications of the laser are the most familiar to us, they are described in more details. They are classified as: Laser at home, which include: 3.4.1 Compact Disc and CD-ROM Optical storage of digital information. Preface Since the beginning of history, man searched for means of storing information, in order to inherit knowledge to following generations. At first the cave-man marked the hunting drawings on the cave walls. Then came shard boards, parchment scrolls, paper, printing, and now the magnetic recordings. Magnetic recordings is used on many devices such as: Tape recorders, computer tapes for storing information in big computers, computer diskettes, and hard drives for storing information in personal computers (pcs). As society developed more, the amount of information is growing at an exponential rate. People are trying to find better ways to store information, and the current trend is toward: 3.4.2 Laser Printer Everyone heard about laser printers, and most offices are using laser printers for printing their documents. We are all aware of the quality of the printing out of a laser printer, but few knows to answers questions about the operation principles of 94 Laser Physics the laser printer: What is the role of the laser in a laser printer? What is the difference between a laser printer and a photocopy machine? Can the same system be used for printing documents from a computer and photocopying documents? What are the advantages of the laser printer compared to the dot printers? The following pages will try to answer such and similar questions by explaining about the laser printer, and the physical principles underlying its operation. 3.4.3 Optical Storage of Information We already saw the Compact-Disc, or CD-ROM in section 9.4.1 as a way to store information and read it optically. There are storage devices which act like magnetic hard disc drives of a computer, but store the information optically. Both writing the information on the optical disc and reading it are done using lasers. These devices allow rewriting information on the optical disk thousands of times, unlike CD, which is write-once device. New devices, which are now at a research stage, are based on holographic writing and reading of information (see chapter 10). These devices store a complete page as an image, unlike the storage of bits in standard storage devices. 3.4.4 optical computer - processing information at the speed of light! Electronic computers are limited by the speed of current flow through the wires inside the computer. By using pulses of light instead of electrical currents it is possible to increase by orders of magnitude the speed of the computers.In electronics, it is possible today to put millions of transistors into one integrated circuit (IC). For optical computers, similar circuits are needed to be developed, and they are called integrated optics (IO). This is a new research subject and there are not yet commercial products of optical computers. In the laboratory, scientists have demonstrated simple operations of edition and multiplication, but it will probably take more than 10-20 years until such products will be available. 3.4.5 Bar code scanner. With increased automation in every-day life, there was a need for a standard auto 95 Laser Physics matic identification system for consumer products. Many automatic systems for identifying products are based on optical systems. Such systems are based on a beam of light, which scan a bar code on the product. The reflected light is read by an optical system. Bar code is a code based on a series of dark and bright bands with specific distances between them. It is made by writing dark bands on white background. Usually the bar code appears on a paper label. In a common bar code the information is coded in one dimension: the width of the dark and bright bands. The length of the bands is just for easy reading and does not contain any meaningful information. 3.4.6 Holograms on credit cards and other valuable products to avoid forgery. Holography is described in chapter 2. Here we shall just mention one of the expanding application of laser in everyday life. Since we now know how to mass-produce holograms that can be seen without using a laser, people are using these holograms in many applications. The production of the master hologram requires sophisticated equipment and special knowledge. This makes them ideal in preventing forgery. The laser is used only in the first production stage of the master hologram Examples for this use of holograms are on:  Every "Visa" credit card.  "Microsoft" software.  Special bank notes. Applications in the future will proobably include all kinds of identification cards. 3.4.7 Optical Fiber Communications Each channel in communications needs a bandwidth (range of frequencies around the central transmission frequency). Optical frequencies (in the visible or Near-Infra-Red spectrum region) are very high frequencies (1014-1015 [Hz]). The bandwidth of voice communication over phone lines is about 10 [kHz]. Thus, the number of phone conversations that can be send over optical communications system is measured in enormous numbers. 96 Laser Physics Diode lasers can be modulated at speeds of tens of Giga-Hertz (1010 [Hz]). and their light can be transmitted over tens of kilometers of optical fibers without the need for amplification. Thus, Optical fiber communications provide the perfect solution for reliable high volume communication. This subject need its own Web site, so we shall just mention here a few facts. Advantages of Optical Fibers:  Wide bandwidth.  Immunity from electrical interference.  Low weight.  Low cost.  More secure transmission. Using optical fibers instead of the metal wires that transmit electrical signal have so many advantages, that all the new communication lines are made of optical fibers. In one optical fiber to the home, all the communications need can be fulfilled: Phone, television, radio, cable TV, computer communication, etc. 3.4.8 Free Space Optical Communications The very high modulation speed of Diode Lasers enables direct line of sight optical communication at very high speed. The main applications of free space optical communications are:  Communication between satellites in space which can transfer information at a bit rates of 1010 bits per second. Thus tens of thousands of phone conversations can be transmitted simultaneously.  Military use of free space optical communication channels are used especially in the battle field, when it is not practical to have fiber optics links. The communication is based on direct line of sight, and provides a secure link because of the very narrow divergence of the laser beam. The advantages of optical communications were described in section 3.4.7. 3.4.9.0 Lasers in Art and entertainment Using lasers that emit in the visible spectrum range, it is possible to create impressive visual effects. When a laser beam pass through a region of humidity, 97 Laser Physics smoke, or any other small particles in the air, the scattered light can be seen by observers from all sides. In big outdoor shows, when the effect need to be seen from a distance, it is possible by moving a small optical element (such as mirror) to move laser beams over large area. For entertainment it is common to use lasers which emit few laser wavelengths. First each color is separated, using prisms, to create many laser beams of different colors. Using small vibrating mirrors, controlled by a computer, it is possible to move each laser beam very rapidly, and create moving colored images. Since our vision is based on seeing the image a little time after it has disappeared, we see a full picture created by laser beam, although the laser beam illuminates each point for a brief period of time. The first devices were used to create two dimentional moving pictures on screens, but the new devices are used to create three dimentional moving sculptures in space (with small particles in it). Using emitted laser powers of few watts, it is possible to create big moving images, in free space, an impossible task to create by other means. 3.4.10 Holograms for exibitions and museums (Details about holography and its applications are described in chapter 10). Holograms allows us to see three dimensional images. Thus there are special holography museums which show holograms as an art by itself. A more advanced use is to show holograms of rare exibits, which can be damaged by exposing to the public. Such exibits include: Archeological exibits which need to be kept at special light, temperature and humidity conditions. Very expensive items, which can be stolen or damage by the public. Rare items which can not be exibited in every museum, but their holograms can. Good hologram contains all the information included in the original object. Once color holography will be developed, many special exibits will be available to be seen at many museums. 3.4.11 Kinetic sculptures. Visible light is used to create visual effects. 98 Laser Physics Using lasers in the visible spectrum, with the help of optical elements which cause reflection refraction, and dispersion, it is possible to create three dimensional sculptures which are moving in space. In order to see the laser beams in space we need a medium which scatter light in all directions. The standard medium is smoke which contains very small particles suspended in the air. When using higher power visible lasers, it is possible to see the reflections from the particles in "standard" air, without the use of smoke. The best lasers for these application are the Argon Ion and the Krypton Ion lasers. A "wall" (plane) of light can be easily created by a rotating or vibrating mirror. By using multiple rotating and/or vibrating mirrors, controlled by computers, it is possible to design complicated shapes which appear in space. 3.5 Scientific/Research Applications 3.5.1 Spectroscopy. Every material has its own characteristic absorption and emission spectrum. By selective excitation using specific wavelengths, it is possible to identify materials with high certainty, even if only small traces exist. Spectroscopy is used in the research of molecules by optically exciting the molecules. It is one of the most important tools in the research of the structure of matter. The laser allows the use of definite controlled wavelengths, which results in a very high resolution measurements. Increasing the accuracy of the determination of the wavelength allows a distinction between smaller details in the material structure. Photo-chemistry is the science of chemical changes which are the result of light. Examples are:  "Tanning" of the skin in the sun light.  Photosynthesis in plants.  The process of vision within the retina cells of the eye.  Induced fluorescence is a very sensitive process, which allows selective excitation of specific energy levels in a s